MESA, Ariz. -- An MRI revealed that Matt Garza has a mild lat strain on his left side, and the right-hander will be shut down for at least one week to make sure he's pain-free.
Cubs general manager Jed Hoyer said the problem area is where Garza's lat and oblique meet.
"It's safe to say it pushes back his first [Cactus League] start, and what it means for the regular season, it's clearly much too early to say," Hoyer said. "We felt [the MRI results were] really good news. It is a mild strain, and we do think it'll be about a week until he should be pain-free, based on the MRI.
"Matt's in good spirits, and he felt much better yesterday," Hoyer said. "We're optimistic, and it was certainly a positive read from our standpoint."
Garza, who was facing batters for the first time since July 21, ended what was supposed to be a 40-pitch session after 20 because of discomfort in his left side. He missed the latter part of last season with a right elbow injury.
Hoyer was behind the batting cage when Garza was pitching. The right-hander stepped off the mound and started stretching.
"It wasn't the most fun live BP session to watch," Hoyer said. "He looked great and free and easy, where you feel really good about what you're watching, and the next thing you know, you see a guy stretching his side.
"Right away, when you see a guy like that stretching, you think the worst," Hoyer said. "It was a bummer, but I'm glad his arm feels good, and Matt's in real good spirits because of that. It's the kind of injury that you're thankful happened in the first live BP. When these injuries happen in the last start of Spring Training, it really hurts you in the season."
Garza, who was 5-7 with a 3.91 ERA in 18 starts last season, was one of the candidates to start Opening Day, but manager Dale Sveum said that decision doesn't have to be made until March 20.
"You can't do anything earlier than that, just because unfortunate things like yesterday might happen and then you'd have to regroup to do different things to get guys set up for the first day, third day of the season," Sveum said. "By [March 20], everybody needs to know what's going on and what their roles are going to be and when they're going to pitch."
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.